[OS] Operating System(1-2): Operation, Resource Management, Virtualization

🍀 운영체제 전공 수업 정리

OS-Operations

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• Bootstrap program - simple code to initialize the system, load the kernel(컴퓨터를 초기화(부팅)하는 역할을 하는 간단한 코드)
• Kernel load
• Starts system daemons
  • 백그라운드에서 실행되는 서비스 프로그램
  • 커널이 로드된 후, system daemons가 실행되며 운영 체제의 주요 기능을 담당
  • 커널 외부에서 제공되는 서비스들로, OS가 정상적으로 동작하도록 지원
• Kernel interrupt driven(기반)
  • Hardware interrupt by one of the devices - 컴퓨터의 device가 CPU에게 작업 요청을 보낼 때 발생
    • ex: 키보드를 누르면 CPU가 이를 감지하고 입력을 처리, 하드디스크에서 데이터를 읽을 때 CPU에게 완료 신호를 보냄
  • Software interrupt (exception or trap)- Software에서 특정한 이벤트가 발생할 경우 CPU가 처리하는 interrupt (동기적임/ ctrl+c 같은 외부 interrupt는 비동기적)
    • software error (ex: Division by Zero, Segmentation Fault)
    • system call - Request for OS service(프로그램이 운영 체제의 기능을 사용하기 위해)
    • ex: 파일 읽기/쓰기(read(), write()), 프로세스 생성(fork()), 메모리 할당(malloc())
  • Other process problems
    • ex: Infinite loop, processes modifying each other or the operating system

Multiprogramming and Multitasking

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1. Multiprogramming (Batch System)
   • Single user cannot keep CPU and I/O devices busy at all times(병원 진료처럼)
   • 여러 개의 작업(code, data)을 관리하여 CPU always has one to execute(CPU가 항상 실행할 작업을 가질 수 있도록 함)
   • 특징:
   1. A subset of total jobs in system is kept in memory
   2. One job selected and run via job scheduling
   3. When it has to wait (for I/O for example), OS switches to another job (다른 작업으로 전환하여 CPU가 놀지 않도록 함)
2. Timesharing (multitasking)
   • logical extension in which CPU switches jobs so frequently that users can interact with each job while it is running, creating interactive computing
   • 특징:
   1. Response time should be < 1 second
   2. Each user has at least one program executing in memory -> process
   3. If several jobs ready to run at the same time -> CPU scheduling (CPU scheduling을 통해 여러 개의 준비된 작업 중 하나를 선택 후 실행)
   4. If processes don’t fit in memory, swapping moves them in and out to run
   5. Virtual memory allows execution of processes not completely in memory

Multiprogramming vs Multitasking 비교

개념	Multiprogramming	Multitasking
방식	하나의 프로그램이 CPU를 점유하면, 다른 프로그램은 대기	CPU가 여러 작업을 빠르게 전환, 동시에 실행되는 것처럼
목적	CPU가 항상 작업을 실행하도록 유지	사용자가 여러 프로그램을 동시에 사용할 수 있도록 지원
CPU 활용	CPU가 놀지 않도록 I/O 작업이 끝날 때까지 다른 작업을 실행	사용자가 실행하는 여러 개의 프로그램을 빠르게 전환
반응 속도	즉각적이지 않음 (배치 처리)	< 1초 (즉각적인 반응)
대표 사례	Batch System (병원 진료 예약)	운영체제에서 여러 프로그램 실행 (Windows, macOS)

• 메모리는 OS 영역, User Process 영역으로 나뉨
• OS는 메모리의 max 주소에서 실행되며, 시스템을 제어하고 관리한다
• 사용자 프로세스(Process 1, 2, 3, 4)는 메모리의 하단부터 위쪽 방향으로 로드됨
  (즉, 프로세스는 주소 0에서 시작하여 위쪽으로 할당됨)
• 다중 프로그래밍 시스템에서는 여러 개의 프로세스가 동시에 메모리에 적재되어 CPU가 작업을 교체하며 실행할 수 있음

Dual-mode and Multimode Operation

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Dual-mode operation allows OS ot protect itself and other system components

• 운영체제를 보호하기 위해 CPU는 두가지 모드에서 작동:
  • User mode: 일반 사용자 응용 프로그램 실행
  • Kernel mode: OS가 실행, 하드웨어 및 시스템 리소스에 대한 전체 권한을 가짐(priviledged mode, supervised mode, system mode)
• Mode bit: provided by hardware
  • Provides ability to distinguis when system is running user code or kernel code
  • 운영체제는 mode bit를 확인하여 시스템 보호를 수행

• Privileged Instruction only executable in kernel mode

• Increasingly CPUs support multi-mode operations
  • Virtual Machine Manager(VMM) 모드 지원
  • VMM has more priviledges than user processes but fewer than the kernel

Transition from User to kernel Mode

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Timer to prevent infinite loop / process hogging resources

• Timer Set: To interrupt the computer after some time period
• Keep Counter: Decremented(감소) by the physical clock
• Operating system set the counter (privileged instruction으로만 가능)
• When counter zero, generate an interrup

• Set up before scheduling process to regain control or terminate program that exceeds allotted time

• 작동 방식
  1. User Mode에서 실행 중인 프로세스가 System Call (ex: 파일 읽기, 네트워크 요청, 메모리 할당 등)
  2. 사용자가 System Call → CPU는 trap 발동 -> Kernel mode로 변경됨(mode bit = 0)
  3. 작업 완료 후 다시 User Mode로 변경됨(mode bit = 1)

mechanism

Resource Management

Process Management

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1. Process란?
   • 실행 중인 프로그램
   • Program은 단순한 코드(passive entity), Process는 실행 중인 코드(active entity)

• Process needs resources to accomplish its task
  • CPU, memory, I/O, files, Initialization data
• Process termination(종료) requires reclaim of any reusable resources
  • 즉, OS는 종료된 process가 자원을 차지하지 않도록 해제해야 함

1. Single-threaded vs Multi-threaded
   |——|——| | Single-threaded | 하나의 프로세스는 하나의 Program Counter 만 가짐 | | ——————- |한 번에 한 개의 명령어만 실행 가능 (순차적 실행) | | Multi-threaded | 하나의 프로세스가 여러 개의 스레드를 가짐 | | —————— |각 스레드는 독립적인 Program Counter를 가지며, 병렬 실행 가능 |

2. Concurrency(동시성)

   • OS는 여러 개의 process를 동시에 실행
   • 일부 프로세스는 User process, 일부는 OS process
   • Concurrency by Multiplexing
     • CPU는 여러 process를 빠르게 Scheduling(전환)하며 실행(동시에 실행하는 것처럼 보이게)

Process Management Activities

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1. Creating and deleting both user and system processes
2. Suspending and resuming processes
3. Providing mechanisms for process synchronization(데이터의 일관성을 위해)
4. Providing mechanisms for process communication
5. Providing mechanisms for deadlock handling

Memory Management

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• To execute a program, all (or part) of the instructions(명령어) must be in memory
• All (or part) of the data that is needed by the program must be in memory
• Memory management determines what is in memory and when
  • Optimizing CPU utilization and computer response to users

✅ Memory management activities

1. memory 사용 추적 * Keeping track of which parts of memory are currently being used and by whom
2. process 관리 * Deciding which processes (or parts thereof) and data to move into and out of memory
3. memory 공간 할당 및 해제 * Allocating and deallocating memory space as needed

File-system Management

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• OS provides uniform, logical view of information storage
  (일관되고 논리적으로 정보를 저장, 관리)
• Abstracts physical properties to logical storage unit - file
• Each medium is controlled by device (ex: disk drive, tape drive)
  • Varying properties:
    1. Access speed
    2. Capacity
    3. Data transfer rate
    4. Access method(sequantial or random)

**File-system management**:

• Files usually organized into directories
• Access control: most systems use access control to determine who can access what
• OS activities include:
  1. Creating and deleting files and directories
  2. Primitives(기본 기능) to manipulate files and directories
  3. Mapping files onto secondary storage
     • Mapping: 사용자가 지정한 파일 이름을 저장장치의 실제 위치로 연결해주는 과정
  4. Backup files onto stable(non-volatile) storage media

Mass-storage management

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📚What is Mass-storage?

• Stores data that cannot fit in main memory or needs to be kept for a long period.
  (i.e. Hard disk, SSD, optical disk, magnatic tapes)

✅ OS management Activities

1. Mounting/Unmounting: Connecting and disconnecting storage devices
2. Free-space Management: Managing available storage space
3. Storage allocation
4. Disk scheduling: Optimizing input/output operations
5. Partitioning: Dividing disks into separate sections
6. Protection

• Optical storage, magnetic tapes are slower but requre management by the OS or applications.

Caching(⭐)

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• Caching is an important concpet applied at multiple levels in a computer(H/W, OS, S/W)

📚 Principle of Caching

• Temporarily copying data from slower to faster storage(cache) for quicker access

✅ Caching works

• When accessing data, the system checks the cache first
  • If found → Use it directly from cache(fast)
  • If not → Copy from slow storage to cache, then use it

📝 Why need Cache Management

• Cache smaller than storage being cached, so efficient management is key
• Design factors:
  • Cache size
  • Replacement policy(Which data to remove when the cache is full)

Type of Storage

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• lower levels(registers, cache) are faster but smaller, while higher levels(SSD, HDD) are slower but larger
• registers are inside the CPU, extremely fast but small
• cache is close to the CPU, faster than main memory but smaller
• main memory(RAM) holds data during program execution
• SSD is faster than HDD but slower than RAM
• magnetic disk(HDD) is the slowest but offers the largest storage capacity\

Data Migration Process

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Data Migration Process

1. Multitasking Environment
   • The system must always use the most recent data, regardless of where it is stored in the storage hierarchy
2. Multiprocessor Environment
   • Requires cache coherency(일관성) to ensure that all CPUs have the most recent data in their cache
3. Distributed Environment
   • The situation is even more complex
   • Multiple copies of the same data can exist, requiring proper management strategies

I/O subsystem

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• OS is to hide peculiarities of hardware devices from the user

✅ Responsibilities of the I/O Subsystem

• Memory Management for I/O:
  1. Buffering: Temporarily stores data while it’s being transferred
  2. Caching: Stores parts of data in faster storage for better performance
  3. Spooling: Manages overlapping of job outputs and inputs by placing I/O data in a buffer
     (It refers to putting data of various I/O jobs in a buffer.)
• Provides a General Device-Driver Interface
  • Ensures that hardware can communicate with the OS
• Drivers for Specific Hardware Devices

Protection and Security

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📚Protection

• Mechanisms for controlling access to processes or resources as defined by the OS

📚Security

• Defense against internal and external attacks Types of attacks include:
  1. denial-of-service(DoS)
  2. worms and viruses
  3. identity Theft
  4. Theft of Service
• User Identification and Management
  • Systems distinguish among users, to determine who can do what
  • User ID
    • Include usernames and unique IDs(one per user)
    • Linked to files and processes for access control(ID는 사용자의 file과 process에 연결되어 access control를 결정)
  • Group ID
    • Allow management of user groups for access rights
    • Associated with processes and files
  • Privilege Escalation(권한 상승)
    • Allows users to switch to an effective ID with higher rights

Virtualization

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📚What is virtualization

• Technology that allows an OS to run applications within other OSes
• ✅ Emulation: Used when the source CPU type is different from the target type (i.e. PowerPC → Intel x86 → M1 / Apple의 Rosetta)
• Generally the slowest method
• If the computer language is not compiled to native code(CPU가 직접 실행할 수 있는 기계어(바이너리 코드)), it uses interpretation(소스 코드를 한 줄씩 해석하고 실행하는 방식)

✅ Virtualization

• OS is natively compiled for the CPU, and it runs guest OSes that are also natively compiled Example:
  • Running Windows XP guests on VMware, with the host OS being Windows XP.
• VMM (Virtual Machine Manager)
  • Software that provides virtualization services (e.g., VMware, VirtualBox).

✅ Use case of virtualization

• Used for exploring and ensuring compatibility(호환성) when running multiple OSes Examples:
  1. Running Mac OS X as the host and Windows as the guest on an Apple laptop
  2. Developing apps for multiple OSes without owning multiple systems → Performing QA testing
  3. Executing and managing compute environments within data centers

✅ Native Execution of VMM

• If VMM runs natively, it acts as the host OS itself
• In this case, there is no general-purpose host OS (ex: VMware ESX, Citrix XenServer) → VMM자체가 host

Computing Environment - virtualization

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Computing Environment - virtualization

• The VMM manages multiple VMs, and each VM operates independently with its own kernel and processes
• This allows multiple OSes to run simultaneously on a single hardware system.
• VMM = Host OS

구분	Computing Environment-Non-virtualized(a)	Computing Environment-Virtualized(b)
Kernel	단일 커널 사용	각 VM이 개별 커널 사용
Process	단일 OS의 프로세스	각 VM이 독립적인 프로세스 실행
Management	하드웨어를 커널이 직접 관리	VMM이 하드웨어 자원을 관리하고 VM에 할당

Distributed Systems

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📚 Distributed Computing

• A collection of separate, possibly heterogeneous systems that are networked together
• Systems typically communicate using TCP/IP Types of networks:
  • 🌐 Local Area Network(LAN): Connects computers in a small
  • 🌍 Wide Area Network(WAN): Covers large geographical areas
  • 🌆 Metropolitan Area Network(MAN)
  • 📱 Personal Area Network(PAN)

📚 Network Operating System

• Provides features to manage interactions b/w systems across a network Key features:
  1. Communication Scheme: Allows systems to exchange messages
  2. Illusion of a Single System: Users perceive the network as a single system

Computing Environment

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Traditional

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1. stand-alone general purpose machines
   • Early computers were stand-alone systems with no internet or network connection
2. But blurred as most systems interconnect with others (i.e. the Internet)
3. Portals
   • Provide **web access to internal systems **
4. Network computers(Thin Clients: 클라이언트 단에서는 최소한의 처리만 수행하고, 주요 작업은 중앙 서버에서 처리)
   • Thin clients act like web terminals, depending on central servers for processing and data
5. Wireless Networks
   • Mobile computers connect with each other using wireless networks
6. Networking Becoming Ubiquitous(보편화)
   • Home systems use firewalls to protect against internet attacks.

✅ Stand-alone → Connected

Mobile

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• Handheld smartphones, tablets, and similar portable devices
• Functional difference from Traditional laptops
  • Extra Features: Mobile OS offer more features like GPS and gyroscope
• Allows new types of apps like augmented reality(AR)
• Connectivity
  • Using IEEE 802.11 wireless networks(Wi-Fi) or cellular data networks(4G, 5G)
• Leaders
  • Apple iOS, Google Andriod

Client-Server

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Older dumb terminals(:서버에서 처리된 데이터를 단순히 출력) have been replaced by smart PCs

📚 Now many systems act as servers, handling requests generated by clients = Client-Server Computing

✅ Types of Servers

1. Computer-server system: Provides an interface for clients to request services, such as accessing databases.
2. File-server system: Provides an interface for clients to store and retrieve files

   Client-Server Connection Structure

Peer-to-Peer(P2P)

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📚 P2P is another type of distributed system where nodes connect directly with each other

P2P

✅ Key feature of P2P

• No distinction b/w clients and servers
• All nodes are treated as peers
• Each node can act as a client, server, or both depending on the situation
• A node must join the P2P network to participate
  → How to Join a P2P Network
  1. Register Service: Nodes register their services with a central lookup service on the network
  2. Broadcast Request: Nodes broadcast service requests and respond to requests using the discovery protocol(네트워크에서 서비스를 찾고 상호 작용을 돕는 protocol)
• Examples of P2P:
  • Napster, Gnutella
  • VoIP(Voice over IP): like Skype

Cloud Computing

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📚 Cloud Computing: Delivers computing, storage, and applications as services over a network

• Cloud computing is a logical extension of virtualization
  • EX: Amazon EC2 - Provides thousands of servers, millions of virtual machines, and massive storage ✅ **Type of Cloud Computing**: 1. 🌐 Public Cloud: Available over the Internet to anyone willing to pay 2. 🏢 Private Cloud: Operated internally by a company for its own use 3. 🔄 Hybrid Cloud: Combines public and private cloud components

✅ **Cloud Service Models**:

1. SaaS(Software as a Service)
   • Delivers software applications via the Internet (i.e. word processor(Google Docs, Microsoft Office 365))
2. PaaS(Platform as a Service)
   • Provides a platform for developers to build and deploy applications (i.e. database server)
3. IaaS(Infrastructure as a Service)
   • Offers infrastructure resources like servers and storage over the Internet(i.e. storage available for backup use)

• Components of Cloud Computing
  • Traditional OSes
  • VMMs (Virtual Machine Managers)
  • Cloud Management Tools
• Internet connectivity requires security tools like firewalls
• Load Balancers: Distribute traffic across multiple applications(to ensure no single server is overloaded)

Cloud Computing

Real-Time Embedded Systems

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📚 Real-Time(실시간) Embedded Systems

• Most prevalent form of computers today

• Use special-purpose OS designed for specific tasks(Ex: Real-Time OS(RTOS))

• Some systems have OS, while others operate without an OS

✅ Feature of Real-Time OS(RTOS)

1. Well-defined, fixed time constraints for processing
2. Processing must be completed within the specified time(Failure to meet deadlines results in system errors)
3. Correct operation only if constraints met